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When passing Ernie's letterbox this morning, I found a courier bag about the size of a shoe-box, resting inside (along with a smaller unlabelled bag). I immediately guessed that it was the special delivery from Acme Industries that Ernie had been muttering about for days - as the contents were listed as **Acme Industries** *1x $Un_2Cl_{14}(H_2O)_7$ single crystal (6 $dm^3$ cuboidal block, irrational edge lengths)*. Unobtanium septa-chlorohydrate, according to Ernie, was the most amazing material known to science. "It has a remarkable crystal structure such that the six faces of a perfect crystal are 'optically asymmetric'". And sure enough, when Ernie slit the bag open and deposited the contents on the breakfast table, it was in the form of a rectangular brick with top, bottom, front, back, left, and right faces respectively coloured silver, bronze, aqua, lavender, gold, and vermillion. But all was not well - Ernie let out a somewhat rude (for him) exclamation and moaned that "the dimensions are all wrong, it's a $1:2:3$ edge-length ratio cuboidal block... and I was promised a $\sqrt{2}:\sqrt{3}:\sqrt{6}$ edge-length ratio block!!!", and simultaneously I noticed that on the packaging there were a number of crudely applied stickers covering some of the letters. I peeled them off to expose the lettering underneath: **Acne Imdustries** *1x $Un_2F_{14}(H_2O)_7$ single crystal (6 $dm^3$ cuboidal block, rational edge lengths)*. Once again, Ernie had mail-ordered a bogus product from Acne Imdustries.

But after spending a bit of time with a set of Vernier calipers, followed by some browsing on a number of online chemical databases, he announced that things were not quite as bad as he had thought. The fluoride (after a quick check with his pocket spectrometer) had exactly the same unit cell dimensions as the chloride, and was also optically asymmetric - perfect for his experiment. The only problem was that the block, whilst the correct volume, had the wrong dimensions. "...and I need those exact dimensions", he explained, "because of reasons".

Fortunately, he thought there was a solution at hand. Ernie had ordered some 'universal crystal glue' at the same time and the smaller bag contained a tiny bottle of exactly that - the labelling confirmed that it was the genuine Acme product. All that was required, Ernie explained, was to cut the $1:2:3$ block up into a number of pieces, and re-assemble it (with the aid of the glue) into a $\sqrt{2}:\sqrt{3}:\sqrt{6}$ cuboid, and then he could run his experiment. "So why don't you do that", he said to me "while I draft a note to the *Mail-order Fraud Investigation Authority*".
[![enter image description here][1]][1]

Ernie told me I would need to use the particle-beam cutter (as most Unobtanium compounds were very hard) but, "not to worry", he had realised that Louisette was a bit of a hazard, so had made a few changes in how the guillotine now worked. So while drafting his letter of complaint he called out a number of instructions:

1. Ernie had improved the beam focus and mounted the cutter on an an
    x-y table with a beam-proof base, so now it could make vanishingly
    narrow, vertically oriented, straight cuts through any material. You
    could program the coordinates of where each cut started and finished
    (in the x-y plane), accurate to the nearest micron.
 2. It was advisable to cut through only one piece of single crystal
    material at a time. The beam could cut through any thickness of
    material, but if you stacked layers there was a risk of 'beam
    refraction' at the interface. (I asked what that meant, and Ernie
    replied that the technical details were a bit complicated, but the
    implication was that the cutting beam could be deflected at right
    angles. As the table was approximately at waist height I decided to
    follow that instruction carefully).
 3. If the glue was painted carefully over two appropriately oriented
    surfaces and the surfaces were pressed together, the glue would bond
    the two pieces and a catalytic process would convert them almost
    instantaneously back into a single crystal - which could now be
    safely cut according to rules 1 and 2 if required. (When I asked what
    'appropriately oriented' meant, he suggested that so long as any two
    pieces being joined retained their original orientations - i.e.
    were not rotated about any axis relative to each other - it
    shouldn't be a problem. But best not to get it wrong or the
    $Un_2F_{14}(H_2O)_7$ might spontaneously decompose into a superheated cloud
    of unobtanium-oxide and hydrofluoric acid. I decided to follow that
    instruction carefully too).
 4. As the glue was fairly expensive (it cost precisely \$10.00 for enough
    glue to join two 1 $dm^2$ surfaces), and Louisette used a lot of
    electricity (it cost precisely \$10.00 for the electricity to make a
    single cut 1 dm long - regardless of the thickness of the cut).
    Ernie would appreciate it if I could make the cutting/joining process
    as economical as possible.

To be honest, I wasn't even sure if it was possible to cut a rational-sided cuboid into an irrational-sided one, and I was so concerned by the fear of disembowelment and/or deflagration that I didn't take any notes of how many cuts I made, or joints I glued, or what order I did them in. But finally I ended up with a single brick of material that looked about the right shape. I handed it to Ernie who, after a few measurements confirmed that he was "fairly pleased" as the block was a single crystal cuboid with the desired $\sqrt{2}$ dm x $\sqrt{3}$ dm x $\sqrt{6}$ dm edge lengths. But he couldn't quite understand why I had chosen only the second-most efficient solution (in terms of cost) rather than the most efficient.

Now I am feeling guilty that I have cost Ernie an unnecessary expense and I am considering buying him something in compensation for the financial loss caused by my clumsy cutting process. Can anybody help me work out how much I owe him?

**Postscript 1:** I still haven't worked out how much I owe Ernie for my error. It's affecting my sleep and I have started having recurring dreams of the conversation I had with him after I "re-formatted" the crystal. In last night's dream I remembered a little more of the conversation and am sure that Ernie definitely stated ***"It's curious that both your solution and my better one, consisted two cuts, followed by gluing the all the pieces back into one piece, followed by two more cuts, followed by gluing those pieces back into the final shape"***

**Postscript 2:** After eating a large plate of cheese last night before retiring to sleep, I had an even more vivid dream of my conversation with Ernie. I'm not sure it will help, but now I recollect that Ernie also said ***"It is nice that the orientations of the crystal lattice planes, relative to the surfaces of the irrational cuboid you have cut, remain the same as the orientations of the crystal lattice planes relative to the surfaces of the original rational cuboid. That results from you keeping as much of the original silver, bronze, aqua, lavender, gold, and vermillion surfaces of the rational cuboid at the surface of your irrational cuboid."***

[1]: https://i.sstatic.net/eJaAY.jpg